I found that x11 works much slower after my recent gentoo update.
I can see that it draws screen much slower that before system update.
I can see it when switching from one window workspace to another. After update the
screen is redrawn much longer. I also can see it when playing movies with
mplayer. The audio is played faster than video what causes that audio and video
is no longer synchronized. Now I get the message from mplayer saying "Your
system is to slow to play this movie". On the other hand glxgears shows the same
results before and after an update but I'm 100% sure that the screen is redrawn
significantly slower.

I performed an update very carefully. I did all the steps suggested by gentoo
packages and "eselect news". I rebuild xf86-video-intel and most probably
turned on what is required in the kernel. My video hardware is intel (815 E) on
board chipset.

And now more details about the system before upgrade and after upgrade.
I upgrade my system twice a year. Previously I upgraded it in summer 2012 and
now I upgraded it at the beginning of 2013.

on my Dell Inspiron 6400 notebook. Now after reading list that you point on, I started to notice some glitch ( out of sync audio/video, sound is faster than video) during some flash video on net. I'll try to downgrade adobe-flash and if this wouldn't help, I'll try to downgrade xorg and play with use flag and we'll see._________________Sorry for my English. It's not my native language and I'm still learning it.

Hi, sorry for the late reply I didn't have much time recantly for these tests. I'll take a look on it at this weekend.

[edit]

So I tried to downgrade xorg-server from 1.13.4 to 1.12.4-r1 and than to 1.11.4-r2 and then I played with xf86-video-intel (2.19.0, 2.20.13, ~2.21.6) but none of these combination stop out of sync flash video, because it's happen only on some pages with flash ( for example not youtube ) I think that problem isn't on my side. But I'll keep monitoring it